1. Field of the Invention:
This invention relates to thermal storage generally, and particularly to a method and apparatus for producing and using low temperature storage ice. While the invention is not limited thereto, it is considered particularly applicable for employment in connection with short-haul delivery trucks which do not have a self-contained mechanical refrigeration system but which carry items requiring a chilled space.
2. Description of Prior Art:
Transportation of perishable products, such as meat for example, generally requires maintaining such products in a chilled condition to prevent undue spoilage. While it is relatively easy to justify economically the provision of large, long haul trailers with self-contained transport refrigeration units, it is harder to justify equipping a fleet of small, local delivery trucks with self-contained mechanical compression refrigeration systems. Also, since self-contained systems are dependent on increasingly more expensive and scarce gasoline or diesel fuel as the energy source, the ever increasing operating costs for such small trucks having a self-contained refrigeration system also enters the picture. Accordingly, it is desirable to develop a practical thermal storage system for these small trucks which would effectively be charged during night periods and/or off-peak electrical power times, and would be used or discharged in truck delivery during the daytime or other peak electrical demand periods.
It is recognized that ice is a desirable thermal storage medium, not only because it is low in cost but largely because, of all possible systems, the water-to-ice phase change normally occurring at 0.degree. C. involves virtually the highest known heat of fusion (being about 79.71 calories per gram). It has been recognized also that water in an anti-freeze solution can be used to form ice, as taught in U.S. Pat. No. 2,101,953. The direct approach to making ice consists in operating a refrigerated evaporator at a low temperature while submerged or in contact with an anti-freeze solution. Problems arise with such an arrangement, however, in that ice formation almost immediately forms and coats the cold evaporator coils and chilling surfaces. Thus further heat exchange or chilling of the anti-freeze solution is greatly impaired unless some means is provided for scraping the ice off the cooling surface, as is disclosed in the noted patent. Alternately, if an anti-freeze solution is chosen which has a very high anti-freeze component level such that no freezing out of ice occurs, then advantage cannot be taken of the high heat absorbed in the phase change involving the conversion of water to ice.
Another known thermal storage system is disclosed in U.S. Pat. No. 2,996,894, which avoids the problems of ice formation upon an evaporator coil by utilizing different density fluids such as oil and water which are immiscible with each other. Here ice is formed by contact of cold chilled oil with the water. In one embodiment the oil is chilled and contacted with water to form ice, thus effecting the thermal storage medium. Subsequently, when utilization of the ice is desired by chilling from the thermal storage reservoir, the oil can be pumped to contact the ice and chilling can occur by virtue of ice melting utilizing the heat of fusion of the ice. In another embodiment in the above patent, disodium phosphate is provided in aqueous solution, with the disodium phosphate precipitating out at a predetermined temperature. Such an arrangement is considered to have the disadvantage of being substantially limited in the extent of depression in temperature (lowering of freezing point of water) that can be accomplished through the precipitation of the disodium phosphates. This is because of the relatively low solubility of disodium phosphate in water.
It is the aim of my invention to avoid the problems associated with prior art arrangements, and in particular to provide a method and apparatus for thermal heat storage employing a high density, low viscosity chilling fluid which, when used, is capable of operation at extremely low temperatures, e.g., as low as -40.degree. F. (-40.degree. C.). Additionally, in its preferred application, the system has been developed for operation in small transport truck an application which permits the thermal storage of ice to be built-up during off-peak power periods.